This invention relates generally to imaging systems capable of operation in multiple modalities, and more particularly to an apparatus and method for correcting multi-modality imaging data.
Multi-modal imaging systems are capable of scanning using different modalities, such as, for example, but not limited to, Positron Emission Tomography (PET), Single photon emission computed tomography (SPECT), and Computed Tomography (CT). Conventional PET/CT imaging systems obtain CT images for attenuation correction in PET and to provide anatomical information to aid a physician in localizing tracer uptake used during PET imaging of a patient. In some cases, the CT diagnostic images are read separately. The accuracy of the reconstructed CT numbers is important to ensure adequate representation of the tracer uptake in the organ. Under normal scanning conditions, this constraint can be easily met by the CT imaging system because of the tight tolerances on the CT number uniformity requirement required of the CT reconstruction algorithm.
In multi-modality systems, for example, an integrated PET/CT system, the PET and CT images should be inherently registered with one another. Inherent registration arises when the detectors for the PET and CT imaging systems are physically mounted to a common frame. Conventional integrated PET/CT systems or SPECT/CT systems utilize data that is generated by the CT imaging system to generate attenuation correction information for the PET or SPECT scan data. Specifically, a plurality of emission attenuation correction factors is derived from CT data that is generated during a CT scan. The term CTAC is used to denote the map of emission attenuation coefficients that are derived from the CT images. The image quality of the CT diagnostic images far exceeds that required to generate the CTAC.
Additionally, patient motion induced imaging artifacts have become an increasingly important issue for PET attenuation correction. For example, since the CT images are typically acquired during a short time period, the attenuation map generated by the CT images represents the attenuation characteristics of the patient where there is little or no breathing motion. In contrast, the PET images are typically acquired over a relatively long time period where a patient is allowed to breathe freely due to the long acquisition time. The mismatch between the two data acquisition modes may result in image artifacts in the attenuation corrected PET images.
One known method for reducing the imaging artifacts is to average the CT image (or generate a maximum intensity CT image) of multiple respiratory phases to mimic the effect of the PET acquisition collected over multiple respiratory cycles. Another known method for reducing the imaging artifacts is to use respiratory gated CT acquisitions to generate attenuation correction maps that better match the respiratory characteristics of the respiratory gated PET acquisition. Typical protocols require a cine CT acquisition of 6-8 seconds with a respiratory motion-monitoring device. CT images of multiple respiratory phases are then reconstructed to match the corresponding phases in the respiratory gated PET acquisition.
Although each of the known methods is effective in reducing patient motion induced image artifacts, each known method also results in an increased X-ray radiation dosage being delivered to the patient as cine-CT scanning requires multiple consecutive exposures to x-ray over some extended period of time. Since, the resolution of the diagnostic CT images far exceeds the resolution required to generate the CTAC one method of reducing the X-ray radiation dosage is to reduce the X-ray tube current, i.e. to reduce the CT image resolution.
For example, when the X-ray tube current is set to a level that is sufficient to generate CT diagnostic images, the electronic noise is a small fraction of the fluctuation due to x-ray photon statistics and, therefore, does not significantly impact the final projection reading. However, when the X-ray tube current is reduced, thus reducing the x-ray flux, the electronic noise becomes a significant portion of the overall noise. As a result, as the X-ray tube current is decreased to reduce patient dosage, significant increases in image shading occurs and the CT number inaccuracies become more apparent. Additionally reducing the X-ray tube current below the level that is used to generate diagnostic CT images eventually results in CT images that are not acceptable for PET attenuation correction.